1. Technical Field of the Invention
The present invention relates to magnetic resonance imaging for obtaining images of blood flow or parenchyma of an object on the basis of a magnetic resonance phenomenon of nuclear spins in the object, and in particular, to a magnetic resonance imaging system and a magnetic resonance imaging method which provide an image whose contrast between blood (or blood flow) and parenchyma is improved by using an MT (magnetization transfer) pulse.
2. Related Art
Magnetic resonance imaging (MRI) is, in general, a technique of applying to an object a radio-frequency (RF) signal at a Larmor frequency so that nuclear spins of the object positioned in a static magnetic field are magnetically excited, and reconstructing an image from MR signals induced responsively to the excitation.
There is a technique of MR angiography for imaging blood vessels as one of the fields of the magnetic resonance imaging. One conventional technique for the MR angiography, which has been put to frequent use in recent years, is to obtain a blood flow image in which blood (blood flow) and parenchyma are contrasted with MT effects (also referred to as MTC (magnetization transfer contrast) effects). One practical example has been proposed by U.S. Pat. No. 5,050,609 (its title is “Magnetization Transfer Contrast and Proton Relaxation and Use thereof in Magnetic Resonance Imaging”).
The research of the MT effects originates from the study of a ST (saturation transfer) method by Forsen & Hoffman (refer to “Forsen et al., Journal of Chemical Physics, Vol. 39(11), pp. 2892–2901 (1963)”). The MT effects are based on chemical exchange and/or cross relaxation between protons of a plurality of types of nuclear pools, such as free water and macromolecules.
As the conventional MR angiography that uses the MT effects, several techniques have been proposed as below.
As is well known, the spectra of protons of free water and macromolecules include the same common frequency range, in which both of the free water of which T2 (spin-spin) relaxation time is longer (T2 is approx. 100 msec) and the macromolecules of which T2 relaxation time is shorter (T2 is approx. 0.1 to 0.2 msec) resonate. Since the T2 relaxation time of a free water signal is longer, its Fourier-transformed signal provides a peak curve of which half-width value is narrow. However, in the case of a signal of protons whose movement is restricted among macromolecules, such as protein, its Fourier-transformed signal provides a broader half-width value, due to a shorter T2 relaxation time. As a result, no distinct peak will appear in the spectrum.
Off-resonance excitation is performed such that, when taking the resonance peak frequency f0 of free water as a center frequency, a frequency-selective pulse serving as the MT pulse is applied to an object to excite a frequency spectrum range shifted by, for example, an amount of 500 Hz from the center frequency f0. This excitation causes the magnetization Hf of the free water and those Hr of macromolecules, both of which have been in equilibrium, to the magnetization Hf of the free water moves to those Hr of the macromolecules. As a result, the value of an MR signal induced from the protons of the free water decreases, while that of an MR signal induced from the protons of the macromolecules also decrease, but at a higher rate. This will cause differences in signal values, region by region, depending on the chemical exchange and/or cross relaxation between the free water and the macromolecules are reflected or not. Those differences lead to differences in contrast between blood flow and parenchyma, thus providing a blood flow image.
At present, the MR angiography based on the MT effects is generally classified into two types of spatially non-selective imaging and slice-selective imaging.
As an example of the former, known is “G. P. Pike, MRM 25, 327–379, 1992”, in which a frequency-selective binomial pulse is used as the MT pulse and applied in a spatially non-selective manner. The contrast between parenchyma and blood flow is obtained according to a relationship of “the MT effects of parenchyma>the MT effects of blood flow.”
On the other hand, as an example of the latter, there is an imaging technique proposed by “M. Miyazaki, MRM 32, 52–59, 1994.” This paper teaches the use of a slice-selective MT pulse composed of both an RF excitation pulse of which duration, that is, a wavelength of the pulse, is longer and gradient spoiler pulses. The application of the MT pulse reduces MR signals emanated from stationary parenchyma of an imaged slice, largely than that from flows of blood that pass the slice, due to its MT effects. The application also reduces the MT effects received by blood flow that comes into the slice, but its reduced degree of a signal from the blood flow is less than that from the parenchyma, so that a certain level of contrast between blood flow and parenchyma is created.
There has also been known a technique of time-of-flight (TOF) angiography conducted in the form of three-dimensional scanning of the head. In this angiography, the duration of an MT pulse is normally specified as being approximately 15 [msec].
Further, concerning MT effects in the multislice imaging, there has been known various reports on “P S Melki and R V Mulkern, Magnetization Transfer Effects in Multislice RARE Sequence”, Magn Reson Med 24, 189–195(1992)”, “A D Elster, Radiology 1994; 190:541–551”, and “D A Finem, Radiology 1994; 190:553–559.” These reports show the use of an MT pulse having an wavelength of 10 to 16 msec.
However, in any of the foregoing various types of MR imaging that adopts the MT pulse, the wavelength of the MT pulse, that is, the duration during which the pulse lasts, is set to a longer value. The duration of the MT pulse occupies, for example, a time of no less than about 35 percent within a single repetition time TR. This longer-duration MT pulse has long been used on the basis of the historical recognition that a shorter-duration MT pulse, that is, its waveform area is insufficient, will limit the MT effects to its lower level.
Thus, to obtain sufficient MT effects one is obliged to make an entire scan time (resulting in an imaging time) longer. In contrast, if performing the multislice imaging is conducted with an MT pulse having a longer duration, with the scan time still unchanged, the number of slices will be reduced.